This invention relates to variable valve action arrangements for lift valves operated by a cam in which a hydraulic chamber is interposed between the valve and the cam.
As disclosed in German Offenlegungsschrifts Nos. 35 32 549 and 38 15 668, variable action lift valves are advantageous because they permit variation, and therefore optimization, of the valve action timing of a machine equipped with the valve, such as a motor vehicle internal combustion engine, as a function of the machine operating parameters even during operation of the machine. In the preferred application of the invention, which is the action for an intake or exhaust valve of an internal combustion engine, variable valve control times and variable valve strokes can be used to influence various engine operating parameters such as: the torque curve by cylinder charge control; crude exhaust emissions, for example, by controlled internal exhaust return; fuel consumption by controlling combustion by way of residual gas content or by decreasing the work required for gas exchange; and brake performance by valve shut-off as described, for example, in German Offenlegungsschrift No. 37 38 556.
In the operation of conventional variable valve action arrangements of this kind, the control valve acts directly on the pressure of a liquid between a plunger and a piston in a hydraulic chamber located between the lift valve and the operating cam. These arrangements provide two extreme positions of the control valve. In one position, the pressure chamber, being closed, constitutes a rigid transmission link. In the other position, the pressure chamber is open to a low-pressure portion of the hydraulic system so that no force can be transmitted by the pressure chamber. With such arrangements, it must be possible to establish predetermined valve lift curves which are reproducible with high precision over considerable lengths of time in operation. Considering, for example, the preferred application, i.e., to control operation of an intake or exhaust valve of an internal combustion engine, maintaining exact preassigned times of valve opening and closing is vital to produce the desired operation of the engine. In this connection, the provision of a valve clearance compensating arrangement is important because it will ensure independence of the lift curve of the valve from wear of the engaging portions of the valve member and the valve seat.